Emacs is easily one of the most powerful (if not the most
powerful) text editors in the world. It has an internal Lisp
programming language, which means that it can be easily extended,
and as a consequence Emacs has suppot for most programming
languages ever invented, as well as a web browser with CSS support
and a world-class news and mail reader.

The user interface is quite unlike most modern editors, but Emacs
comes with internal documentation which can help you out.

This is an old SGML mode which has been patched to support
XML. It reads the DTD, can use an external parser to validate
documents, does syntax colouring as well as help you insert only
the correct elements at your current location.